CheckFortranSourceCompiles: F90 by default

CheckFortranSourceCompiles defaults to fixed-format Fortran 77, which makes testers put six blank columns in front of every line. This surprising default should be changed to the free-format text of Fortran 90 and newer, which does not have the surprising punch-card days need of six spaces starting every code line. Fortran 2018 standard deprecates fixed-format code, another reason to update the default to F90. Finally, the reason one would use this function would most often be to check if a compiler can handle new language features, which are going to be in free-format style.